Viral Gastroenteritis: This is a common condition that can cause nausea, especially after eating, due to inflammation of the stomach and intestines. The acute onset and association with meals make it a plausible diagnosis.

Food Poisoning: Consuming contaminated or spoiled food can lead to nausea after meals. The recent onset and meal-related symptoms support this possibility.
Gastroesophageal Reflux Disease (GERD): Although more common in adults, GERD can occur in teenagers, causing nausea after eating due to stomach acid reflux.
Irritable Bowel Syndrome (IBS): While IBS is more chronic, its symptoms can include nausea after meals, especially if the patient has a sensitive stomach or reacts to certain foods.

Appendicitis: Although nausea is a non-specific symptom, appendicitis can sometimes present with nausea after meals, especially if the appendix is located near the stomach or intestines, causing referred pain or irritation.
Pregnancy: In a female patient, it's crucial to consider pregnancy as a cause of nausea, especially if the nausea is persistent and occurs at any time, not just after meals.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can cause nausea after eating due to chronic inflammation of the gastrointestinal tract.

Gastric Outlet Obstruction: A rare condition where the stomach outlet is blocked, which can cause severe nausea after meals.
Cyclic Vomiting Syndrome: A disorder characterized by recurrent episodes of severe nausea and vomiting, which can be triggered by eating.
Eating Disorders: Conditions like bulimia nervosa can lead to nausea after meals due to purging behaviors or other psychological factors affecting eating habits.